Silica micro-encapsulation (SME) is a new technology that has significant economic and environmental value because it allows disposal of oil- and/or hydrocarbon-contaminated drill cuttings, either onshore or offshore, in an environmentally safe manner.

SME is a process by which the oil attached to the drill cuttings is removed by emulsification and then physically encapsulated within an insoluble matrix of amorphous silica. The process consists of the addition of an emulsifier in an acidic environment, followed by the addition of water-soluble silicate.

Specific emulsifiers are selected to cover hydrophilic-lipophilic balance (HLB) requirements for a broad range of oils used in drilling fluids, including synthetics, mineral oil, diesel, crude oil, and bitumen.

Laboratory and pilot tests of the SME technology show that, using environmentally acceptable emulsifiers, oil-based mud present on drill cuttings is removed and forms a stable oil-in-water emulsion. These results led to development of a process for SME treatment of drill cuttings that could be environmentally acceptable for offshore applications, as described in previous publications.1,2

Introduction

Offshore and onshore disposal of drill cuttings generated with synthetic and oil muds is prohibited in some areas of the world. As a result, the drilling industry has moved to second-generation synthetic-based muds (SBM's). However, in some areas, such as the North Sea, governmental authorities continue to change the legislation to the point that they now have under consideration a maximum allowable discharge of less than 1% oil. This regulatory action is a consequence of the environmental impact of oil-wet drill-cutting accumulations on the ocean floor ecosystem, even with SBM's that pass all required environmental tests.

Various chemical processes have been applied to convert these drill cuttings to an acceptable form so that the associated SBM or oil-based mud does not cause an environmental problem. However, the majority of prior art processes have logistic limitations. For example, one patented process uses emulsifiers to remove the contaminant oil from the cuttings;3 this is fine, but one still faces the logistic problem of handling wastewater and oil waste on offshore rigs.

SME is an onsite process that removes oil from the cuttings in the form of microscopic droplets and then encapsulates the oil droplets within an inert material. The two-step chemical process can be described as:

Application of an emulsifier, in an acidic environment, to the hydrocarbon- or oil-contaminated drill cuttings. This emulsifies the oil into microscopic droplets (less than 10 microns).

Application of reactive alkaline silicate to the emulsified oil. An instantaneous acid-base reaction occurs between the reactive silicate and the acid, producing a solid amorphous silica shell that surrounds and traps the micron-size oil droplets.

Oil-in-water emulsions were prepared and evaluated using blends of surfactants with various HLB index values to determine the required emulsifier package for each type of oil. To select optimum treatment levels of the additives, the concentrations of acid, emulsifier, and reactive silicate were varied and the end product evaluated. The effect of oil/water ratio was also evaluated to determine the minimum amount of water necessary to obtain good emulsification.

The efficiency of the encapsulation process was evaluated by placing the treated cuttings in synthetic seawater to better simulate offshore conditions. The synthetic sea-water contains 41.9 g of sea-salt ASTM D-1141-52 per liter of water. Free, unencapsulated oil can be qualitatively detected visually with an oil-soluble dye. The amount of unencapsulated oil can be quantified with liquid/liquid extraction, followed by gas chromatography/mass spectrometry (GC/MS).

Development of Silica Micro-Encapsulation Technology

Emulsifier Package Selection.

The first step in the development of this technology was the selection of emulsifier packages for each type of oil used in several basic drilling-fluid systems.

Oil-in-water emulsions were prepared and evaluated using blends of anionic and nonionic surfactants with various HLB index values to determine the required emulsifier package for each type of oil. Each individual oil has a specific HLB value at which emulsification is optimum. The HLB of an emulsifier is related to the ratio of the size and strength of the hydrophilic (or polar) and the lipophilic (or nonpolar) groups of the molecule.4,5

Emulsifier packages used in the treatment of contaminated cuttings were selected based on the resulting average droplet size of the hydrocarbon- or oil-based mud-in-water emulsion. Weighting material was omitted from the oil-mud formulations, owing to particle-size overlap with the droplet size of the emulsion. Oil-droplet sizes were measured after each minute of mixing, for up to 10 minutes. The criteria for average droplet size of the oil-based mud-in-water emulsion was less than 10 microns, after 3 to 5 minutes of mixing time, followed by no significant change in droplet size.

The average droplet size of the oil-in-water emulsion was measured using a light-scattering technique with a Malvern Instruments Mastersizer. Emulsion samples were dispersed in water following the procedure recommended by Malvern Instruments. Data analyses were based on the volumetric average droplet size.6

Emulsion stability was determined by measuring emulsion volume, free oil caused by coalescence, and free water caused by creaming over an 8-week period. Coalescence is defined as the irreversible union of two or more emulsion droplets to produce a larger drop with a lower interfacial area.7,8 Creaming refers to the mutual attachment of individual emulsion drops without the loss of integrity of the individual drop;8 it occurs in almost all emulsion systems in which there is a difference in the densities of the two phases.

The emulsifier packages that generated the least amount of free oil and excess water over time, and had lower droplet sizes, were selected for the encapsulation process.

Treatment of Contaminated Drill Cuttings: Laboratory Scale.

Oil-contaminated cuttings, in batch sizes of 300 g or less and containing various oil concentrations, were treated using a Hamilton Beach mixer at low shear. The schematic diagram of the treatment process (Fig. 1) shows a first stage that involves the application of an emulsifier (in an acidic environment below pH 2) to the hydrocarbon- or oil-contaminated cuttings. The second stage involves the addition of reactive alkaline silicate to the emulsified oil, inducing an instantaneous acid/base reaction between the reactive silicate and the acid, producing solid amorphous silica that surrounds and traps the micron-sized oil drops.